The race for the Republican presidential nomination in Iowa turned theological over the weekend, with one of the three front-runners, Marco Rubio, seeking to capitalise on an approving comment from evangelical commentator Russell Moore.

Rubio, Donald Trump and Ted Cruz regard capturing the evangelical constituency as key to their success in the state. The most recent poll puts Cruz ahead of Trump on 28 per cent to Trump's 24, with Rubio in third place with 13 per cent. The other candidates have struggled for traction.

Now a statement by Moore, president of the Ethics and Religious Liberty Commission of the Southern Baptist Convention, has ruffled feathers among Cruz and Trump campaigners after it neatly encapsulated their respective followers in terms of high-profile evangelical figures.

Rubio's staff sent an email quoting Moore's comment that the candidates are "appealing to disparate camps of evangelicals". Moore said in a Roll Call interview on Friday: "'I would say that Ted Cruz is leading in the Jerry Falwell wing, Marco Rubio is leading the Billy Graham wing and Trump is leading the Jimmy Swaggart wing."

Jerry Falwell is associated with the uncompromising Moral Majority movement of the 1980s that helped sweep Ronald Reagan to power. It campaigned on issues including opposition to homosexuality and abortion, traditional family values and support for Christianity in the public sphere. Falwell's son Jerry Junior is the president of Liberty University who recently caused outrage with his call for students to carry concealed weapons on campus.

Jimmy Swaggart is a Pentecostal televangelist and singer whose ministry imploded after a series of sex scandals.

Billy Graham is theologically conservative, but made a point throughout his active ministry of reaching out to those who did not share his position.

Moore appears to be implying that Rubio is a less divisive and hard-line figure than Cruz (who was famously described as a "wacko bird" by Republican elder statesman John McCain after a Senate filibuster). Among Cruz's policies are the immediate repeal of Obamacare and tightening abortion laws. He is pro-gun, anti-immigration and does not believe human beings are responsible for climate change. 

Ironically, his approach is more similar to that of Graham's son Franklin than that of any of the other candidates. 

In a sign of rising tensions ahead of the February 1 Iowa poll, Cruz is fending off attacks on his right to stand in the election at all. Under the Constitution, presidents must be "natural born citizens"; Cruz's mother was American, but he was born in Canada. Donald Trump has said this makes his candidacy questionable.

Asked on Fox News Sunday whether he really doubted Cruz was a natural born citizen, Trump said, "I don't know. I really don't know. It depends.

"Does natural born mean born to the land, meaning born on the land? In that case, he's not."

Another Republican candidate, Senator Rand Paul of Kentucky, said on CBS' Face the Nation it was unclear if Cruz qualified as a natural born citizen.

In response, Cruz said: "Three weeks ago, almost every Republican candidate was attacking Donald Trump. Today almost every Republican candidate is attacking me. And that kinda suggests something has changed in the race."